Otago University Zoology academic and dolphin researcher Professor Liz Slooten said there are instances of dolphins coming to the aid of humans when sharks are nearby, but it depends on the size of the shark.

She said a dolphin might not get involved if it were dangerous to do so.

Dolphins' sonar allows them to see mammals' distress and recognise the need to breath air, which explains stories of dolphins pushing people - and in a recent case, a dog - back to shore when they're having trouble in the water.

Ms Slooten said smaller species of dolphins, such as the Māui dolphin, would be unlikely to intervene if a human was faced with a shark.

However, she said, most of the time humans are around sharks nothing happens.